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Last edited by Refractingoptician.com; 02-28-2007 at 04:58 PM.
This question has been asked and answered many times lately. You make a Franklin bifocal. Grind two lenses for each eye, Split them and mount them in the frame. This is the cheapest, quickest and least expensive approach, as well as being the most opticaly correct. 2nd alternative contact one of the specialty labs and have them custom make lenses.
Chip
Potential problems: You must be very carefull splitting and mounting lenses. I reommend a table disc sander with fine grit for the last couple millimeters finished with a fine hand flie.
If the person reads only a little, one might try a bifocal lens only in the dominate eye for spot reading.
or
Make a clip-on (that can also be rotated upwards out of view) with prism to neutralize the distance prism for reading.
or
a second pair of sepctacles with or without a bifocal for reading only, ignoring the distance prism. The person can always close one eye for a short time.
or
a creative choice that you and your customer can design.
May be cheaper yet to buy a Fresnel, split it in half and apply it above the bi-line. Just a suggestion, (which I stole from someone else). :D
Make 2 pairs :bbg:
Much more profitable!:cheers:
Epic Labs does a great job on orders like thse! Their number is 320-656-1473.
Cassandra
I'm not as well versed as I should be on "specials" grinding, but a straight-top bifocal should be able to be ground in a similar manner used in bicentric or "slab-off" grinding, but grinding the prisim in the distance instead of the near.
The problem with using Fresnel's is that they will drop visual acuity at least a line. Most would find that unaccepable. If it were me, I would be going with 2 pair.......However Chip's solution would work as well.
Chip, I REALLY have to come down and watch you work someday!:D ( proving you are never to old to learn!)
hj
"Always laugh when you can. It is a cheap medicine"
Lord Byron
Take a photo tour of Cape Cod and the Islands!
www.capecodphotoalbum.com
Ok, Now I want to know how that can be achieved using a slab. Is this possible?
Jane,
You actually hit the problem on the "head" :) most of these guys in this thread, kind of "jumped" the gun and started down the slab road.. the prism wanted was ..hmm base out..while slab is only correcting imbalance in the vertical merridian :)
I think a Ben Franklin is a bit of an over kill here (cost wise)..Fresnal (optically) stinks) especially in this small of a degree of correction, the loss of accuity out weighs the correction.
The cheapest way to go about this is use a little optical theory (Prentice rule) .. figure out how much prism you need and cut it on a FT45 where the add OC is on center..say you wanted wanted two out in the distant only you grind 3.5 out decenter the near OC to the correct PD and it will end up with 2 in only the distant :).. I usually cut it slightly (the prism) to allow for the distant power Vs. the total add power. and taking into account prescribed prism vs. resultant prism.
Theoretically if you wanted to pay more for a lens you could also get a lens with base in in the near..move it out to the correct PD pushing out the distant (neutralizing the near and placing it into the distant) Than again a ribbon add or a prism seg is more costly compared to "cheating" with a FT45 :)
Just tossing out idea's..don't shoot me...
Jeff "I think I'm full of lab tricks, my wife just says FULL of something" Trail
Ok, now I need a visual! Wouldn't it be easier to use an Executive? By the time you've ground your prism at distance and decentered to neutralize in the near, you only have a few mm of near w/o any prism before you've run out of bifocal.
:D This is fun!
Or or or or , Use an OD for an OS and vice versa, then use the DVI calculator to figure prism at near then decenter out.
Still in serious need of a visual.
Man my brain hurts, I've been thinking about this WAAy too hard.
Last edited by Jane; 03-26-2003 at 01:18 PM.
THAT is why you use a FT45 blank vs. a smaller width add... you have more room to play with, so now it ends up looking like a FT35 or a FT28 depending on the movement (PD,frame size etc., etc.)Jane said:
Ok, now I need a visual! Wouldn't it be easier to use an Executive? By the time you've ground your prism at distance and decentered to neutralize in the near, you only have a few mm of near w/o any prism before you've run out of bifocal.
:D This is fun!
An E-Line will work but cosmetically most people would prefer a FT over the E-Line where the seg insets (nasal/temple) and due to design are top heavy in thickness and since we are using prism can't tinker with any prism thinning tricks .. depending on the frame size and PD and prism direction you can tinker with other size FT and take advantage of the molded inset, and if you REALLY want to put your brain into a SPIN-A-ROO.. I have done RD22's and 24 bifocals where you cut the incorrect axis while surfacing further off than usual and then end up with the correct axis after manipulating the design through over or under rotation but NOT actually moving the seg location keeping the correct PD and seg hght.
Jeff"who says you can't teach an old dog new tricks..as long as you got the right doggie treats" Trail :)
Could always use a glass round 30 seg which are avaiable with prism controlled segs....
You would need a pair with 4 prism IN each eye to cancel out the distance prism.
A work around for the problem of running out of seg with a FT45 is to decenter for 3 prism diopters instead of four. Instead of decentering an extra 16mm leaving 6.5mm of temporal viewing area, decenter 12mm leaving 10.5mm. You are left with one prism diopter base out which although not desirable, may be tolerated. A second pair for prolonged near task would be ideal regardless of the final solution.
Another possibility is to grind two separate SV lenses, one for the distance with the prism an no prism for the near. Cut the lenses in half and bond the distance to the near. Looks like an executive lens. Chip, is this what you were describing when you said "make a franklin (exec) bifocal? This would probably provide the best and most comfortable vision of all the possible solutions (except two pair). Epic can also make a lens with just about any amount of prism in the seg that is required (neutralizing the distance prism).
I think the clip-on idear would be more feasable. and if you did the lens with the rx bifocal without the base out prism, then grind plano 4 out prism lenses into a matching clip-on to be worn for distance, to be removed when reading; also much lest costly and complicated if the dr. decides to change the prism power in the near future, could even make the clip-on transitions, ha ha.
I was actually thinking quite the opposite. I was assuming that the clip-on could be attached during a reading session and allow the spectacles to be unincumbered for general distance viewing.
If a small amount of reading was required while the clip-on was not available, closing one eye would allow the task to be accomplished.
The clip-on could be carried for special situations requiring extended reading. For sure it would still be best to have a pair of reading glasses, with no prism, for longer periods of near work , perhaps in additon to the clip-on.
Erich. I had considered that aspect, but, what I was thinking,#1. if you did it that way, you'd have to grind the prism in both sets of lenses, and #2. the risk of a dr. power change is high, so if the dr. did change the rx prism, you'd have to change both set of lenses, now if you can convince the pt to take that financial risk, instead of you, then fine, do it that way...
Sorry I didn't get into this conversation a little earlier.
Gary,
The RX you presented is akin to many of the jobs we have running through the lab at this very moment. Using Prentice rule and decentering to achieve desired prism is a very good way to go IF the prism amount can be achieved in this manner. The two fundamental problems in doing it this way is that 1) you end up with a segment size that does not correlate with the customer's request. Depending on the add power and prism needed, some jobs like this can end up with a segment that looks like a D35 (or for that matter a D31, D37 or some other odd segment).
2) The other fundamental problem with this is that it is not a universal rule. Even using a D45 does not solve the problem where the add power is low and prism amounts can't be achieved by decentration.
It has been my experience that a prism segment (that is a D28, D35, etc.) is the most universal solution for these types of RXs. Optically it is the only solution where you are not "cheating" the patient into wearing something they didn't really ask for.
:finger:
As for Lenny's solution about making two pairs because of the profitability:
Just remember, we are a SERVICE industry. If it were your eyes I don't think you'd want to change glasses every time you wanted to read a label at a grocery store or read the program at a baseball game.
epiclabs. I thought that the problem was that the pt needed the prism in the DISTANCE, so how would a Prism - seg help that?
epiclabs: Of course separate readers is a very legitmate option .... to be used for that purpose. The grocery store (spot reading) is handled by simply closing one eye.
Last edited by Homer; 04-25-2003 at 07:58 AM.
I have actually had some work done through Epic labs, and what they have done for my patients, is to put prism base in in the seg. Effectively cancelling out the base out prism in only the near.
The end result looked close to a slab. And everyone one of my patients have been happy :) Including the retired engineer!
Ranger,
In this case where distance prism is base out we would do one of two things. The first option would be to make a base in prism seg to cancel distance prism effectively giving you zero near prism while maintaining your base out distance prism. The second option would be to do a Franklin style split which Jeff mentioned earlier in this thread. You take a pair of single vision lenses and grind 4 diopters of prism base out OU. Then you take a pair of flat-top bifocals and grind your RX with zero prism. Now you cut each lens across the middle and cement them together at the "split" line to create your glasses.
Jubilee,
I believe the lenses you described are the Franklin lenses and not the actual prism segs. If they have a horizontal "split" line or slab-like line then that's what they are. Thickness concerns or material index may have been the reason for that choice (you'd have to refresh my memory):D
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